The gene regulation relating to malignancy that was studied included primate retrovirus expression, and modulation of human chorionic gonadotropin, and placental alkaline phosphatase. The effort to induce novel primate retroviral expression utilizes transfection by early region SV 40 DNA of cell strains to obtain permanent indicator lines to overcome host range restriction and for use in retroviral induction, and use of Rhesus monkey and human trophoblast in cell and organ culture as targets for retroviral isolation. Improved methods have been developed for growth of human placenta in organ culture and for isolation of trophoblast cells for growth in monolayer culture. Rhesus placental tissue, but not other Rhesus fetal organs, contain MAC-1 p26 expression, and this persists in trophoblast cell and organ culture. A Rhesus trophoblast cell strain transfected by SV 40 Gene A displays prolonged growth in cell culture, and expression of placental alkaline phosphatase activity, type IV collagen, laminin, fibronectin and by EM contain desmosomes, but no p26 activity.